U.S. Pat. No. 2,935,199 issued to Willinger and owned by the assignee of this application, discloses an in-tank aquarium filtration system of the under-gravel variety. The system includes an apertured platform in spaced relation from the bottom of the aquarium tank for defining a space between the platform and the tank bottom. Gravel is distributed over the platform, the apertures in the platform being sufficiently small to prevent the gravel from falling therethrough. A hole is provided in one corner of the platform, and means are provided about the hole for securing a filter assembly. The filter assembly comprises two spaced apart tubular members connected at their lower ends to a housing defining a chamber for providing communication between the tubular members. A charcoal filled filter cartridge is disposed on the upper end of one tubular member, which serves as a lift tube, and an air supply is connected to the upper end of the other tubular member, which serves as an air tube.
When the filter assembly is secured on the platform above the hole therein, the hole establishes communication between the filter assembly and the space beneath the platform. Upon activation of the air supply, suction is established in the chamber at the bottom of the filter assembly. This suction draws aquarium water through the gravel bed into the space beneath the platform, and from there through the hole in the platform into the filter assembly. The water is then drawn up into the lift tube as the air bubbles from the air tube rise therein. Upon reaching the top of the lift tube, the water passes into the filter cartridge, which is apertured for this purpose. After passing through the cartridge, water is recirculated back into the aquarium via apertures in the top of the cartridge.
While the arrangement disclosed in the Willinger patent suits the intended purpose, the construction of the filter assembly comprising, as it does, two spaced apart tubes connected at their lower ends by a housing, is both cumbersome and space consuming. Another under-gravel filtration system of the type including a filter assembly comprising two spaced apart tubular members connected at their lower ends, is disclosed in U.S. Pat. No. 3,199,678. That patent is also issued to Willinger and owned by the assignee herein.